1. Field of the Invention
This invention relates to an optical recording medium for recording information by laser, relates etc. and reproducing the recorded information, more particularly to an optical recording medium which is capable of optical recording and reproduction at high density by a laser having wavelengths in the visible and near-infrared region and also low energy.
2. Description of the Prior Art
Generally speaking, an optical disc is capable of memorizing high density information with a number of optically detectable small pits (e.g., about 1.mu.) in the form of a helical or circular track, each pit being formed on a thin recording layer provided on a substrate. For writing information in such a disc, a focused laser beam is scanned on the surface of a laser-sensitive layer, and only the surface irradiated with the laser beam forms, as stated, pits, which are formed in the form of a helical or circular track. The laser-sensitive layer is capable of forming optically detectable pits by absorption of the laser energy. For example, according to a heat mode recording system, the laser-sensitive layer absorbs heat energy and forms small recess portions (pits) through vaporization or deformation at the sites where heat energy has been absorbed. Alternatively, according to another heat recording system, through absorption of the laser energy irradiated, there can be formed portions differing in oxidation degree, reflectance or optical density created by chemical changes at the irradiated portions, the difference being optically detectable.
The information recorded in the optical recording medium is detected by scanning a laser along the track and reading the optical changes at the portions where pits are formed and the portions where no pit is formed.
As the recording medium to be used for such recording-reproduction, there have heretofore been known metal thin films such as a vapor deposited film of aluminum, film composed principally of inorganic materials such as a bismuth thin film, tellurium oxide thin film or calcogenite type amorphous glass film, etc. or films of organic materials such as a plastic coated film containing a dye (pigment).
However, according to the methods of the prior art as described above wherein pits are formed by chemical changes, the sensitivity is insufficient and, moreover, the optical contrast between the pit-forming portions and the non-pit-forming portions is small, thus involving the disadvantage that the information recording does not have a high S/N ratio. Further, the recording medium having recorded information has the drawback of lacking stability in storage over a long term. On the other hand, according to the method wherein holes are formed, while high sensitivity and a high S/N ratio can be obtained, there is involved the drawback that the surface of the laser-sensitive layer exposed in the air may be oxidized in an environment having moisture or an oxidative atmosphere, to be deficient in stability. For overcoming such drawbacks, it has been proposed in the prior art to provide a protective layer on the upper and/or lower side of the laser-sensitive layer. Such a protective layer, however, will contrariwise lower sensitivity and S/N ratio.